Is it permissible for a woman to travel in a medical convoy without a mahram, given the presence of safe companionship, to provide medical and social assistance to remote villages, knowing that the presence of a mahram is unattainable?

The charitable acts performed by the convoy, which you mentioned, do not justify a woman traveling without a mahram (definition). The texts of Islamic law are explicit in prohibiting a woman from traveling without a mahram, such as the Prophet's (peace be upon him) saying: "A woman should not travel except with a mahram." Safe companionship does not permit this, except in the case of the obligatory Hajj according to some scholars. Your work is supererogatory and not obligatory, and avoiding prohibitions takes precedence over fulfilling commands, based on his (peace be upon him) saying: "When I forbid you from something, avoid it; and when I command you to do something, do as much of it as you can." Therefore, we do not see this journey as permissible for you.